Stop setting USE_PYTHON3 for jobs
Stop setting USE_PYTHON3 for jobs

DevStack is now hardcoded to always use python3, so we can drop this
setting from the zuul config.

[0] Ieffda4edea9cc19484c04420ed703f7141ef9f15

Change-Id: If4292a649e2d61c2d1cb7e28cd7ee593d6a62d9c

openstack/barbican,openstack/barbican